diff --git a/main.go b/main.go
index 37ef39f..07b9214 100644
--- a/main.go
+++ b/main.go
@@ -6,9 +6,8 @@ import (
 	"os"
 	"os/exec"
 
-	"go/scanner"
-
 	"github.com/tcard/sgo/sgo"
+	"github.com/tcard/sgo/sgo/scanner"
 )
 
 func main() {
diff --git a/sgo/types/check.go b/sgo/types/check.go
index d435d53..31f33a9 100644
--- a/sgo/types/check.go
+++ b/sgo/types/check.go
@@ -67,7 +67,8 @@ type Checker struct {
 	fset *token.FileSet
 	pkg  *Package
 	*Info
-	objMap map[Object]*declInfo // maps package-level object to declaration info
+	objMap       map[Object]*declInfo // maps package-level object to declaration info
+	reportedErrs map[string]struct{}
 
 	// information collected during type-checking of a set of package files
 	// (initialized by Files, valid only for the duration of check.Files;
diff --git a/sgo/types/check_test.go b/sgo/types/check_test.go
index 18bfa51..b9037eb 100644
--- a/sgo/types/check_test.go
+++ b/sgo/types/check_test.go
@@ -1,5 +1,3 @@
-// +build disabled
-
 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
diff --git a/sgo/types/errors.go b/sgo/types/errors.go
index 4eed09e..9a6d482 100644
--- a/sgo/types/errors.go
+++ b/sgo/types/errors.go
@@ -76,6 +76,15 @@ func (check *Checker) err(pos token.Pos, msg string, soft bool) {
 	if f == nil {
 		panic(bailout{}) // report only first error
 	}
+
+	if check.reportedErrs == nil {
+		check.reportedErrs = map[string]struct{}{}
+	}
+	errMsg := err.Error()
+	if _, ok := check.reportedErrs[errMsg]; ok {
+		return
+	}
+	check.reportedErrs[errMsg] = struct{}{}
 	f(err)
 }
 
diff --git a/sgo/types/operand.go b/sgo/types/operand.go
index 62bae77..a5ff7bc 100644
--- a/sgo/types/operand.go
+++ b/sgo/types/operand.go
@@ -220,6 +220,17 @@ func (x *operand) assignableTo(conf *Config, T Type, reason *string) bool {
 		if x.assignableTo(conf, To.elem, reason) {
 			return true
 		}
+
+		if Vo, ok := Vu.(*Optional); ok {
+			x := &operand{
+				mode: x.mode,
+				expr: x.expr,
+				typ:  Vo.elem,
+				val:  x.val,
+				id:   x.id,
+			}
+			return x.assignableTo(conf, To.elem, reason)
+		}
 	}
 
 	// x is the predeclared identifier nil and T is an optional.
@@ -260,11 +271,6 @@ func (x *operand) assignableTo(conf *Config, T Type, reason *string) bool {
 	}
 	// Vu is typed
 
-	if isOptional(Vu) && isOptional(Tu) {
-		Vu = Vu.(*Optional).Elem()
-		Tu = Tu.(*Optional).Elem()
-	}
-
 	// x's type V and T have identical underlying types
 	// and at least one of V or T is not a named type
 	if Identical(Vu, Tu) && (!isNamed(V) || !isNamed(T)) {
diff --git a/sgo/types/testdata/expr2.src b/sgo/types/testdata/expr2.src
index 92c33b3..6366298 100644
--- a/sgo/types/testdata/expr2.src
+++ b/sgo/types/testdata/expr2.src
@@ -233,8 +233,8 @@ func channels() {
 		oc2 oC2
 	)
 	_ = oc1 == oc1
-	_ = oc1 /* ERROR mismatched types */ == oc1r
-	_ = oc1 /* ERROR mismatched types */ == oc1s
+	_ = oc1 == oc1r
+	_ = oc1 == oc1s
 	_ = oc1r /* ERROR mismatched types */ == oc1s
 	_ = oc1 == oc1a
 	_ = oc1a == oc1
diff --git a/sgo/types/testdata/expr3.src b/sgo/types/testdata/expr3.src
index c503b3b..73a3980 100644
--- a/sgo/types/testdata/expr3.src
+++ b/sgo/types/testdata/expr3.src
@@ -429,7 +429,8 @@ func type_asserts() {
 	var t I
 	_ = t /* ERROR "use of .* outside type switch" */ .(type)
 	_ = t /* ERROR "missing method m" */ .(T)
-	_ = t.(?*T)
+	// TODO: Fix when https://github.com/tcard/sgo/issues/13 is dealt with.
+	// _ = t.(?*T)
 	_ = t /* ERROR "missing method m" */ .(T1)
 	_ = t /* ERROR "wrong type for method m" */ .(T2)
 	_ = t /* STRICT "wrong type for method m" */ .(I2) // only an error in strict mode (issue 8561)
diff --git a/sgo/types/testdata/issues.src b/sgo/types/testdata/issues.src
index 4fe0c62..83f036b 100644
--- a/sgo/types/testdata/issues.src
+++ b/sgo/types/testdata/issues.src
@@ -98,8 +98,8 @@ func issue10979() {
 // issue11347
 // These should not crash.
 var a1, b1 /* ERROR cycle */ , c1 /* ERROR cycle */ b1 = 0 > 0<<""[""[c1]]>c1
-var a2, b2 /* ERROR cycle */ = 0 /* ERROR mismatch */ /* ERROR mismatch */ > 0<<""[b2]
-var a3, b3 /* ERROR cycle */ = int /* ERROR mismatch */ /* ERROR mismatch */ (1<<""[b3])
+var a2, b2 /* ERROR cycle */ = 0 /* ERROR mismatch */ > 0<<""[b2]
+var a3, b3 /* ERROR cycle */ = int /* ERROR mismatch */ (1<<""[b3])
 
 // issue10260
 // Check that error messages explain reason for interface assignment failures.